1. Field of the Invention
The present invention relates to infrared-radiation detectors, and, more particularly, to uncooled bolometers for detection of infrared radiation.
2. Description of the Related Art
Detection of infrared radiation emitted by warm bodies provides an important method for night vision (perception without visible light). Infrared detectors may be classified in various ways, such as scanning or staring arrays, cryogenic (typically liquid nitrogen temperatures) or uncooled detectors, 3-5 micron or 8-12 micron spectral sensitivity range, and photon or thermal detection mechanism. Cryogenic infrared detectors are typically made of small bandgap (about 0.1-0.2 eV) semiconductors such as HgCdTe and operate as photodiodes or photocapacitors by photon absorption to produce electron-hole pairs. For example, see U.S. Pat. No. 4,686,373 (Tew et al) which describes a hybrid system with HgCdTe photocapacitors glued to silicon signal processing circuitry.
Uncooled infrared detectors cannot make use of small bandgap semiconductors because the bandgap is only about 4kT at room temperature and dark current swamps any signal. Consequently, uncooled infrared detectors rely on the other physical phenomena and are less sensitive than cryogenic detectors but do not require cooling apparatus or its energy consumption. For portable, low power applications where the greater detectivity of cryogenic detectors is not needed, the preferred choice is an uncooled thermal detector: infrared photons are absorbed and the resultant heating of the absorbing element is detected. The thermal detector is usually one of three types: (1) pyroelectric detector, (2) thermocouple, or (3) bolometer.
The pyroelectric detector uses a ferroelectric ceramic material (such as BaSrTiO.sub.3) at operating temperatures (typically 0 C. to 150 C.) a little below its Curie temperature. The preferred ferroelectric materials have a large change in spontaneous dielectric polarization at operating temperatures, and the heating of the ferroelectric is detected by sensing the induced voltage created by the generation of charge across a capacitor with the ferroelectric as insulator. For example, see U.S. Pat. Nos. 4,348,611 (Ruppel et al), 4,142,207 (McCormack et al), and 4,379,232 (Hopper).
The pyroelectric detector is a hybrid approach that has problems when extended to large detector arrays (arrays of 256 by 256 pixels) due to ferroelectric material defects, contact defects, and bump bonding defects which imply low yields.
Thermocouples rely on the change in contact potential of a junction of dissimilar conductors with temperature; see for example G. Lahiji et al, A Monolithic Thermopile Detector Fabricated Using Integrated-Circuit Technology, 1980 IEEE IEDM Tech.Dig. 676 which uses bismuthantimony or polysilicon-gold couples in monolithic arrays on silicon.
Bolometers typically rely on the temperature change of resistance of thermally isolated thin metal films or semiconductor films. The thin films may be fabricated on a suspended dielectric film in a silicon substrate and may be located adjacent to monolithic detection circuitry on the silicon substrate. The dielectric film is thermally isolated from the remainder of the silicon substrate by etching away the silicon from beneath the dielectric film so that it is suspended. See for example K. C. Liddiard, 24 Infrared Physics 57 (1984) and 26 Infrared Physics 43 (1986) which includes amorphous silicon films for resistance change with temperature plus abutting nickel films for infrared absorption and electrical contact. The foregoing references are hereby incorporated by reference.
Thermocouples and bolometers avoid the yield problem of the pyroelectric detectors because they can be monolithically fabricated on a silicon wafer along with the detection circuitry. However, thermocouples and bolometers have the problem of lower detectivities than pyroelectric detectors.